I've never had a problem with a buyer that couldn't be solved so I have no good advice for you but I thought I would do a little research since one never knows when the next customer will open a case - so what to do..
Here is what i found on Etsy Help
https://www.etsy.com/help/article/364?ref=help_homeLooks like you have done much of what Etsy says in their policy. I would continue to email Etsy with all the back up regarding the case that you can provide - perhaps directly from this help page - as it might get you to a different email inbox.
You also might consider emailing a team leader on one of the teams you belong to (such as Etsy Success) and see if they will be able to help at all. I hope you are able to resolve this quickly,
Good luck